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G90 piston ring clearance

Old 09-01-2011, 08:30 AM
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singlechannel
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Default G90 piston ring clearance

I have fitted a new ring on my 10 year old G90. The liner glaze was lightly removed with 600 wet and dry. Although it runs okay the compression is definately not very good. power is down a little and idle is up a little.
In its day it was very reliable, easy starting, good idle and power but now its lost that healthy plop it used to have when it was flicked over.
My question is what should the gap at the ends of the piston ring be? When I fitted this new ring it was 6 thou but should it be nearer 2 or 3 thou. I am assuming that since this new ring has now had just over an hours running it will have sealed itself near as well as it ever will. Apart from the gap which will have clearly have grown alittle.
Since I hav'nt got much to lose with this motor I could probably close the ring gap a few thou by gentle tapping or peaning over the very ends of the ring in an attempt in regain some of the lost compression.
Thanks
Old 09-01-2011, 03:32 PM
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tigrejohn
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Default RE: G90 piston ring clearance

Don't attempt to rework the ring. Usually the metal used is cast or wrought and is brittle. If you try to "move" metal, you will end up removing it. If you have the old ring, try this: spread the ends of the ring apart. It won't take much for that ring to snap. It will not bend. If you want to find out if the end gap is too much, contact Frank Bowman (renown aftermarket ring manufacturer) and ask him.
Old 09-02-2011, 05:44 AM
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Bax
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Default RE: G90 piston ring clearance

The ring gap when the engine is fully assembled will be visible. It will be about 1/64" or so, and t his allows for expansion of the ring when the engine gets up to operating temperature. Piston rings are made of very brittle material, so if you try to rework one, you'll only wind up breaking it. It's not very workable at all.

Usually, when the ring gap is too large from wear, it's the ring that's worn and not the cylinder sleeve. We've honed sleeves with brake cyliner hones in the past, but only very lightly. If you use 600-grit or finer sandpaper, you won't remove enough material to matter. Compression loss is more likely through a bad fit of the ring to the piston than a result of a bad cylinder-to-ring fit.
Old 09-02-2011, 06:54 AM
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singlechannel
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Default RE: G90 piston ring clearance

Thanks for your reply it was very informative. The fit of the ring in the piston groove is obviously much looser than it was when new so as you say some compression is probably being lost around the inside of the ring. I think its worth a running for a while longer as it can't get any worse and it may well get better. Thanks again, Rob
Old 09-03-2011, 09:28 PM
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oskartek
 
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Default RE: G90 piston ring clearance

You may consider a new piston.
Although it is presently on back order.
Tower now has international shipping.

http://www3.towerhobbies.com/cgi-bin...?&I=LXFY56&P=Z


Happy flying, Oscar

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